1. Field of the Invention
The invention pertains to the field of a conversion device. More particularly, the invention pertains to a heat-power conversion magnetism device.
2. Description of the Related Art
Converting thermal energy into power had a long period of time in human history. Especially the converting thermal energy into electrical power is the most common way to produce energy source today. However, the efficiency of energy conversion is still very low. For example, the efficiency is about 40% for steam power plant and about 30% for internal combustion engine. Almost 60-70% of energy is wasted in other words. After power conversion, the remained energy becomes too low so that the temperature thereof is usually below 200° C. and is mostly below 100° C. Such kind of “low-grade” energy cannot be utilized by most of the thermal engine available today.
Taking the solar energy as an example, the sunshine provides a huge amount of energy to earth, but the energy density is one KW per square meter only. Flat panel of sunshine collector can convert solar energy into thermal energy very efficiently (>90%) and cost effectively, but the thermal energy density is relative low. Usually the temperature of a flat panel solar hot water system is likely below 100° C.
To convert low grade of energy (<100° C.) into useful mechanical power by appropriate utilization of the magneto caloric effect (MCE) of solid ferromagnetic materials is desirable.
MCE has discovered for over 100 years. Emil Gabriel Warburg discovered the MCE in the iron in 1881. Soon, after Warburg's discovery, Edison and Tesla have tried to convert power from the MCE of soft iron by a loop of heating and cooling, as disclosed in U.S. Pat. No. 396,121, U.S. Pat. No. 428,057 and U.S. Pat. No. 476,983. For a very long period of time, such technology was only applied in very low temperature refrigeration to cool down samples at few Kelvin to tens Kelvin since 1930's. For near room temperature applications, magnetic refrigeration was not able to achieve until 1976. Gadolinium (Gd) has been used as a magneto cooling working material and demonstrated the magnetic refrigeration at near room temperature in 1976. Gd, which has a Curie point of 293 Kelvin, is used as a magneto cooling working material by G. V. Brown of National Aeronautics and Space Administration. The temperature change of 14° K. has been produced by applying 7 T magnetic field. Since then the study of the application of MCE materials has been increased, a lot of MCE material properties can be found in the disclosure of K. A. Gschneidner Jr, V. K. Pecharsky, “Recent developments in magnetocaloric materials”, Institute of Physics Rublishing, Reports on Progress in Physics, Rep. Prog. Phys. 68 (2005) 1479-1539.
In 1997, V. K. Pecharsky and K. A. Gschneidner discovered that the entropy change (DS) of Gd5(SixGe1-x)4 is much larger then Gd in near room temperature, and the Curie temperature thereof can be changed from 29 Kelvin to 290 Kelvin by changing the composition of silicon(Si) and Ge. Gd5(SixGe1-x)4 can be a desirable magneto caloric effect material (MCEM).
The basic principle of magneto caloric effect can be used as the magnetic refrigeration (or heat pump), which is disclosed in Peter W. Egolf, Andrej Kitanovski, “An introduction to magnetic refrigeration”, University of Applied Sciences of Western Switzerland; C. Zimm, A. Jastrab, “Description and Performance of a Near-Room Temperature Magnetic Refrigerator”, Advances in Cryogenic Engineer, Vol. 43; and G. V. Brown, “Magnetic heat pumping near room temperature”, Journal of Applied Physics, Vol. 47, No. 8, August 1976, also we can find the basic theory of magnetic cooling in the disclosure Andrej Kitanovski, Peter W. Egolf, “Thermodynamics of magnetic refrigeration”, International Journal of Refrigeration 29 (2006) 3-21.
For an reversible adiabatic process and from Maxwell equation, the equations are disclosed as the following:ΔSm=−∫μ0(∂M/∂T)dH  (1)wherein Sm is magnetic entropy; μ0 is a permeability factor of vacuum; M is magnetic moment; T is temperature; and H is magnetic field strength;ΔTad=−∫(T/Cp)(∂M/∂T)dH  (2)wherein ΔTad is adiabatic temperature change, and Cp is heat capacity; andΔSm*T=Cp*ΔTad   (3)
When a magnetic field is applied to the MCEM and the MCEM is magnetized, the magnetic entropy, Sm, is changed according to the magnetic field changing due to the magnetic order of the material is changed. Under the adiabatic condition, the magnetic entropy change, ΔSm, must be compensated by an opposite change of the entropy associated with the lattice. The result is a change in temperature of the MCEM. In other words, when a magnetic field is applied to MCEM and the MCEM thus loses its magnetic entropy, the temperature of the MCEM rises up to compensate the magnetic entropy loss. When the magnetic field is removed away from the MCEM and the MCEM thus increases its magnetic entropy, the temperature of the MCEM cools down to compensate the magnetic entropy increase.
By using MCEM with proper thermal dynamic cycles, some heat engine for cooling, or for heating, can be designed for a better performance.
There are four basic processes for MCE magnetic heat engine: (A) adiabatic magnetization: a MCEM is subjected to a magnetic field in adiabatic condition, and the temperature of the material rises up then; (B) constant magnetic field cooling: a cold thermal heat source is provided to cool the material down to a lower temperature; (C) adiabatic demagnetization: the magnetic field is removed away from the material in an adiabatic condition, and temperature of the material goes down accordingly; and (D) zero magnetic field heat absorption: a hot thermal heat source is provided to warm up the material. For a cooling application, the process (D) is used to cool down the environment. For a heating application, the process (B) is used to warm up the environment.
From this equation, we can know that the magnetic entropy change of MCEM is relative to the (∂M/∂T). The larger (∂M/∂T) of the material is, the larger entropy change will be, which will induce larger cooling capacity of magnetic thermal cycles. For the magnetic cooling (heat pump) application, the magnetic field is chosen to change the magnetic phase of MCEM, and the result is the change of magnetic entropy and eventually the change of temperature. The more largely the magnetic moment changes, the larger cooling capacity will be achieved.
When dealing with heat-power conversion, the thermal energy is chosen to change the magnetic moment of MCEM, and the result of conversion is the power generation. For most of the MCEM, as the heat is applied to the MCEM and the temperature passes through the Curie temperature, the magnetic moment of MCEM will change from a high to a low value. Assume that a magnetism device with MCEM has been designed for the magnetic flux flowing through the MCEM. When the thermal energy is applied to the MCEM and changes its magnetic moment, the magnetic flux will be changed due to the magnetic moment change.
The magneto caloric effect material (MCEM) is not only suitable for a magnetic refrigeration application, it is also suitable for the reverse processing such as a heat-power conversion application.
U.S. Pat. Nos. 396,121, 428,057 and 476,983 show the earlier ideas of heat-power conversion device. Although those prior arts give some great ideas of how to change the thermal energy into the mechanical energy or electrical power, it never comes to realization. It requires huge amount of energy to rise the temperature up to the Curie temperature, and the converting efficiency is low. Simply because of the near room temperature, MCEMs had not been discovered until 1970's. U.S. Pat. No. 396,121 also requires spring or flywheel as mechanical energy storage device to complete the thermal cycle. Also, the armature moves forward and backward like a pendulum, which is not an efficient way for power generation.
Both U.S. Pat. No. 428,057 and U.S. Pat. No. 476,983 require electric conductor coil for electrical power generation. When the temperature of the magnetic core is changed around its Curie temperature, the magnetic moment will be changed and thus cause the magnetic flux to change, thus the induced electrical current flows through the electric conductor coil. Another report in the disclosure of Paul F. Massier, C. P. Bankston, ECUT, Energy Conversion and Utilization Technologies Program “Direct Conversion Technology”, Annual Summary Report CY1988, Dec. 1, 1988a also introduces electric conductor coil for electrical power generation. The problem of electric conductor coil is that the power generation of the coil strongly depends on the magnetic flux changing frequency. The thermal transfer process for changing the magnetic moment of MCEM is a slow procedure, and the cycle time is 6 seconds (0.166 Hz) in the report of Reference of C. Zimm, A. Jastrab, “Description and Performance of a Near-Room Temperature Magnetic Refrigerator”, Advances in Cryogenic Engineer, Vol. 43. Another report of Dr. Zimm of Astronautics Corporation shows the operation frequency of 4 Hz. Being under such a low operation frequency will limit the electrical power output of electric conductor coil and waste large amount of MCEM for converting enough power.
U.S. Pat. No. 4,447,736 discloses the rotary magneto caloric ring system schematic. In this system, the MCEM is formed in a ring shape, and the MCEM is rotated around the center of the ring shape. An extra magnet covers certain portion of the ring, and a hot heat exchanger and a cold heat exchanger are applied to the rotating ring. A part of the rotation ring is being heated by the hot heat exchanger, and a part of the rotation ring which is outside the magnetic field bounds is being cooled. The cooled portion of rotary magneto caloric ring, which the temperature is under its Curie temperature, will be attracted by the magnetic field. Such kind of rotary magneto caloric ring system schematic can provide a continuous and smooth mechanical torque output. However, it is difficult to utilize all the magnetic flux generated by the magnet, and only a part of the magneto caloric ring is attracted by the magnet, thus the mechanical torque output is relative low. Also, how to prevent the leakage of the refrigerant between the heat exchanger and the rotary magneto caloric ring became a very difficult issue.
Some interesting magnetic cooling or heating devices for generating a thermal flux with magneto caloric materials are disclosed in US2007/0130960 and US2008/0236172. Permanent magnets are used to generate magnetic field, and multiple number of MCEMs are used so as to subject them to a variation in magnetic field. In order to generate the strongest magnetic field as possible, a number of MCEMs are arranged as a multiple number of magnets. The location arrangement of magnetic poles of magnet and the MCEMs are well alliance, and the magnetic flux can pass through the magnetic paths as smoothly as possible. In other words, the magnetic resistance of magnetic paths is designed to be minimized. For the first example of US2007/0130960, twelve thermal bodies made of MCEM and six magnetic elements are used. Such arrangement can allow the minimum magnetic resistance and maximum magnetic flux passing through the MCEMs when they are allied. Although such arrangement can create the maximum thermal effect of the MCEMs, but it also leads to extra problems. The static torque is still large and requires more driving power to move the magnetic field away from the MCEMs.
Both U.S. Pat. No. 6,668,560 and U.S. Pat. No. 6,935,121 show a rotating magnet magnetic refrigerator. Each of a number of magneto caloric materials is a common multiple of the magnetic poles, and the attraction force at neutral position is large, thus the torque required to overcome the attraction force is large.
An MCEM is temperature dependency of magnetization. Cleber Santiago Alves, Sergio Gama, “Giant Magnetocaloric effect in Gd5(Si2Ge2) Alloy with Low Purity Gd” and E. Bruck, O. Tegus, “Magnetic refrigeration—towards room-temperature application”, Physica B 327 (2003) 431-437 show the magnetization curves of Gd, Gd5Si2Ge2, and MnFe(P,As) at near room temperature.
FIG. 1. shows magnetization curves of Gadolinium (Gd); FIG. 2. shows magnetization curves of Gd5Si2Ge2; FIG. 3 shows magnetization curves of (Mn, Fe)2P0.5As0.5; and FIG. 4 shows MCE of MnFePAs in 2 T and 5 T magnetic field.
The materials in FIGS. 2 and 3 show the dramatically magnetic moment change when the temperature of the materials changes around its Curie temperature (Tc). Such kinds of materials are perfectly suitable for heat to mechanical power conversion. FIG. 4. shows the entropy change calculated using the equation (1).
When an MCEM is subjected to a magnetic field, a huge magnetic property (magnetic moment) change occurrs over relatively small temperature change near the Curie temperature. Referring to FIG. 4, it is much clear to understand how the magnetic phase changes corresponding to the temperature. At 2 T magnetic field strength, the magnetic phase changes completely when the changing temperature (around 12 Kelvin) between T10 and Thigh.
If a heat source temperature is 10 Kelvin higher then Tc, it will be enough to change the magnetic moment from high to low. Taking the FIG. 4 for an example, the Curie temperature of the material is 280 Kelvin, the hot heat source of 290 Kelvin and cold heat source of 275 Kelvin will be enough to change the magnetic moment between Thigh and Tlow.
Such kind of hot heat source can be found everywhere in our ordinary life. The disclosure of Andrej Kitanovski, Marc Diebold, “Applications of Magnetic “Power Production: and its assessment”, Final Report, Swiss Federal Office of Energy—BFE, 2007 shows some of the type of heat source, for example solar, geothermal, vehicle or industry processes, and the temperature range from 60° C. to 180° C. The invention intends to convert. such low-grade thermal energy into useful mechanical power efficiently.